weird shaking/unsteady drive

[JoeFish]

G'day,

I have a Manual 98 VT executive commodore and i have a weird problem that is kind of hard to explain.

I recently got new spark plug leads because my car was struggling when i accelerated as it would jump and skip when it was going up in speed. When i got the leads replaced, it felt as if it was fully fixed, it wasn't jumping at all. But now,( a few weeks later) the problem has reoccurred where i'll be in (for example) 4th gear, and doing 70km/h (on a steady highway on the sunshine coast) and even though me foot is still on the accelerator, the car is struggling to drive smoothly, for it jumps and shakes forward and backwards a bit. It is rather hard to explain. It feels like the car is trying to drive at the requested speed but it can't, so it tries do drive faster or slower but can't do neither, reason for it bouncing around.

Sorry about the poor explanation, but it is hard to explain. Could it be my spark plug coils? The mechanic said that if the leads didn't fix my problem the 1st time, it would most probably be the coils.

Any advice, please type it here

Cheers,
Joe

[Tsunamix]

COils yes but lets look a little broader.

Need 3 things to run well. Fuel, Spark and air.

Fuel - check your fuel filter and fuel pump. Fuel filters can gunge up and choke off the supply of fuel so that when you demand more fuel - acceleration and high speed cruise, you get an uneven run.

Spark - irregular misses can be caused by anything in the spark chain. most commonly it's due to a flashover to earth. Any place where the spark can follow a different path to negative than through the sparkplug can cause this. One of the classics is a split coil, so that the spark shorts to the support bracket instead of firing down the lead and into the plug.

Air - - well you dont often get many problems with that so all you really have to do is check your air cleaner.

Now this is where it gets tough - In electronically controlled engines, there can be many little electronic things that affect the above 3 simple categories in unexpected ways.

For example - if your Throttle position sensor (TPS) is buggered, you may be doing 70kph with a constant throttle opening, but suddently the engine things the throttle is closed and you want to decelerate. So it cuts off the suply of fuel to the engine and the engine leans out and misfires.

There are lots of these little sensors that can do this. TPS, Crank angle sensor (governs the spark timing), and anti knock sensor (detects pre ignition and retards the timing) are all worth checking. Look for splits in cases, busted leads, disconnects etc.

Indirectly, little things like bad joins in the harness, partly unclipped connectors and the like can do it as well.

I think I would give the engine bay a really really good clean when cold. Degreaser and steam. Then I would check those 3 sensors and every connection and joiner you can see. I would bet that you have a buggered CAS or a split coil pack as my top 2 choices though.

[JoeFish]

Thanks heaps for your in depth reply man

I have got a new fuel filter, mechanic thought that would fix it BEFORE i got the spark plug leads redone.

I checked the airfilter box etc, i happened to find a small bird in it a few weeks ago but that made no change haha.

I'll deffinantly get the Coils checked out and also the TPS etc.

Thanks heaps for the advice!!

[Maxseverity]

sounds like what happened when my fuel pump blew coz the filter was almost blocked.

[JoeFish]

my friend just serviced my car and went through it all and he said that i have small hole in my exhaust near the converter. By the way i have a straight through exhaust which can you see in my pics in my profile.

He said something about my car having two intakes from the converter to the exhaust...something like that, i can't really remember so i'm sorry if i'm not making any sense.

He also said something about the sensors could be sending incorrect signals because of this, which might cause the skipping and jumps.

Sorry for my poor knowledge about cars lol...and bad memory.